In the Impact Character Throughline, the Symptom is where this character hopes to have the greatest impact, and Response is how he wants things to change because of that impact. Could you explain this for me in the context of preparing the story, because this is essentially the root of understanding the IC Symptom and Response regardless if they are Steadfast or Change, correct?
The idea of "how [the IC] wants things to change because of his impact" presupposes several things: a(that the IC is aware of the MC, which it needn't be, b) that the IC is aware of its influence on the MC, which it needn't be, and c) that the IC is trying to influence the MC to change, which it needn't be trying to do.
The answer to your question is "no, it is not the root of understanding the IC Symptom and response..." It is ONE understanding, and looking at it that way does not take into account the temporal nature of a story where the ebb and flow of influence waxes and wanes and builds (or decreases) as the story moves forward. You're looking for a spatial relationship between the story points to answer a temporal process. They are interconnected, temporally, spatially, and in the context of the other throughlines -- all of which describe the evolving/devolving effects of the inequity at the center of the story. Generalizations, like the one quoted above serve to clarify complex relationships, inadequately represent the entirety of those relationships and therefore fall short of providing "root understanding" for anything.
My recommendation about this whole IC business and your writing is to let it go. You're aware that there is an IC and you know the nature of the context within which the audience will view the IC. That is sufficient to write your story well enough that your audience should put the pieces (and connections) together for themselves.
Think of the four throughlines like composing a sentence. One throughline is a noun, another a verb, another an adjective, and the last like an adverb. Whichever noun, verb, adjective, and adverb you put in the sentence, readers glean meaning through both their understanding of the meanings of the vocabulary you choose, but also by your vocabulary's inherent relationship to one another because of the type of grammatical family into which each falls. You don't need to tell your audience that an adjective acts upon or moderates a noun because that is part of what makes an adjective an adjective.
You do not need to tell the audience that an IC influences or impacts the MC because that is built into the nature of the IC as defined by a storyform. The storyform holds the grammar and nature of the narrative (sentence) structure. You, as author, choose WHICH "noun", "verb", "adjective", and "adverb" fit within the storyform you've chosen, conform to the nature of the narrative elements, and illustrate your creative style and intent through your storyencoding and storyweaving. It is up to
the audience, through Story Reception, to unweave and decode your work to find the underlying storyform that indicates your story's underlying meaning.
Once you've written your first draft, you can test to see if an audience understands if the IC is seen as the IC in the story. If it isn't, you may choose to be more explicit in illustrating the IC story points and how they influence the MC, but I recommend letting your muse guide you through your first go at writing the story. There are plenty of opportunities to
make adjusts during rewriting.